1. Field of the Invention
The invention described in the present specification relates to a display panel having self-luminous elements arranged in the form of a matrix on the panel, and a panel module having a driving circuit mounted in the display panel. In the present specification, the display panel and the panel module will both be referred to as an emissive type display device. In addition, the invention in the present specification has aspects of a semiconductor device, an electronic device, and a power supply line driving method.
2. Description of the Related Art
One of basic performance requirements of a display is brightness (luminance). Therefore, recent displays (for example liquid crystal displays, plasma displays, and organic EL (Electro Luminescence) displays) are expected naturally to have high luminance irrespective of difference in display system.
On the other hand, a display that emits light at a maximum luminance at all times has a problem of being too bright and glaring rather than offering high performance. Further, this kind of display consumes a large amount of power, and is inferior in terms of environmental performance.
Accordingly, a method of properly using a maximum luminance (peak luminance) and average luminance (all-white luminance) is employed for displays. This method has been used since displays of a cathode-ray tube type were mainstream.
However, a controlling method of the cathode-ray tube type display greatly differs from the recent displays due to differences in light emission principles and driving method.
In the case of a plasma display, for example, the maximum luminance and the average luminance are controlled by securing a wide dynamic range of video signal level. In the case of a liquid crystal display, on the other hand, the maximum luminance and the average luminance are controlled by controlling the brightness of a backlight separately from a video signal (that is, by controlling the maximum luminance and the average luminance by two parameters of the video signal and the backlight).
In addition, a case where a display is mounted in a portable device operating using a battery as a power source needs to be taken into consideration for this luminance control. The portable device in this case includes not only devices providing display as a main function but also devices combined with an information processing function and a communicating function.
The portable device is desired to have a mode of changing display luminance according to the brightness of the ambience and a power saving mode intended for use for a long period of time.
Further, the portable device is desired to have provisions for both a high-luminance mode assuming outdoor use and a low luminance mode to be seen naturally even when used in the dark.